Primary productivity in the central equatorial Pacific (3°S 130°W) during GasEx‐2001

Abstract

Measurements of chlorophyll concentration, phytoplankton productivity, and nutrient dynamics were made during the GasEx‐2001 cruise to the equatorial Pacific in February 2001. During the core measurement period of the experiment, a parcel of water was tracked over a 16‐day period in order to close the mixed layer carbon budget. Chlorophyll concentration averaged 0.16 mg m−3 and integrated mixed layer primary productivity increased from 10 to 50 mmolC m−2 d−1, concomitant with a shoaling of the thermocline. The mean f ratio (ratio of new to primary production) decreased from 0.17 at the surface to 0.04 at 60 m, and the ratio of silicate to nitrate uptake by phytoplankton was 0.8. These results are close to or slightly lower than climatological values, and are consistent with moderate productivity in an iron‐silicate co‐limited environment. Fast repetition rate (FRR) fluorometry indicated that the effects of iron limitation increased with distance from the equator, the origin of upwelled iron. Measured chlorophyll concentrations were used to calculate the attenuation of solar irradiance, which facilitated a more accurate simulation of mixed layer temperature in a separate physical study. The productivity data presented here were incorporated into a carbon budget of the mixed layer, which derived a gas transfer velocity in excellent agreement with direct CO2 flux measurements.